This invention relates to the conversion of chemical energy to electrical energy and more particularly to a process for producing a primary cell which comprises lithium as an active metal anode and an iodine-containing cathode material.
One area of use for such electrochemical cells is for providing electrical power to inaccessible devices such as implanted cardiac pacemakers. However, they may also be applicable to a wide variety of devices requiring primary batteries intended to provide relatively high voltage and high energy density over long life under low current drain conditions.
Such cells may be constructed, to form electrolytes in situ. For example, when the electrochemically active ingredients are lithium and iodine, a solid lithium iodide electrolyte forms between the anode and cathode after the cell has been constructed. Alternatively, the electrolyte can be preformed in whole or in part. For example, one might make use of the following arrangement: Li/LiI(Al.sub.2 O.sub.3)/PEO.nI.sub.2. Such an arrangement might be desirable for modifying a self-discharge rate. Such alumina dispersions are reported in C. C. Liang, J. Electrochem. Soc. 120, pg 1289 (1973); C. C. Liang and L. H. Barnette, J. Electrochem. Soc. 123, pg 453 (1976), and Liang U.S. Pat. No. 3,713,897, all of which are incorporated herein by reference.
Another modification which may be incorporated in these cells as a part of the operative relationship thereof is the use of an anode coated with poly(2-vinylpyridine) (P2VP) or other polymeric material such as described in the U.S. Mead et al., U.S. Pat. No. 3,957,533 or a self-supporting poly(2-vinylpyridine) body such as described in the U.S. Skarstad, U.S. Pat. No. 4,182,798. The subject matter of these patents is incorporated herein by reference. I.sub.2 /P2VP cathodes according to this invention when used with such lithium/P2VP anodes provide a battery of unusually high rate capability for this type of cell.
After the fabrication of the cell, the solid electrolyte layer continuously grows through the above mentioned self-discharge reaction due to the diffusion of iodine from the cathode through the layer. With the growth of the layer, the internal resistance of the cell increases proportionally to the square root of the storage period of the cell, reflecting the increasing difficulty of the iodine diffusion through the layer.
This formation of the layer, however, does not necessarily occur uniformly over the entire lithium surface. Particularly when the cell is stored at a high temperature, such as at 45.degree. C. or 60.degree. C. for example, the lithium anode is consumed locally intensively by the reaction with the iodine, consequently forming pits in the lithium surface. Such a phenomenon is often observed in metallic corrosion and is called pitting corrosion.
The intensively corroded portions of the lithium anode are substantially inactive to cell discharge reaction due to a thick electrolyte layer formed thereon and the effective area of the lithium anode is thereby reduced. When the cell thus stored is subjected to the cell discharge, the slope of the discharge curve of the cell becomes steep in comparison with that obtained immediately after the fabrication of the cell, reflecting reduction in the effective area of the lithium anode. Also, significant variation in cell-to-cell voltage and resistance is developed during self-discharge.
In U.S. Pat. No. 4,332,865 issued to Sotomura, et. al., which is incorporated herein by reference, this problem is addressed in a solid electrolyte cell by subjecting the cell, immediately after its fabrication, to preliminary cell discharge until the amount of the discharge reaches at least 2 mAh per cm.sup.2 of the lithium anode surface, or alternatively the cell is subjected to self-discharge at a temperature of not more than 30.degree. C. until the amount of the self-discharge reaches at least 3 mAh per cm.sup.2 of the lithium anode surface. However, we have found that in certain cells, such immediate predischarge provides a cell with higher than desired impedance and greater than desired voltage variation during initial discharge.
The objects of the present invention are therefore to provide an improved process for producing a primary cell in which variation in cell voltage and impedance are reduced and in which self-discharge during storage is slower and more consistent.